Stress Corrosion Cracking of Low Temperature Sensitized AA5083

Non-heat treatable Al-Mg alloys are desired for modern marine systems due to light weight, moderate strength, corrosion resistance, and weldability.  When exposed to temperatures above 50^oC for extended periods of time, Al-Mg alloys (e.g., 5083-H131) become susceptible to intergranular corrosion (IGC) and stress corrosion cracking (SCC).  The threat to structural integrity, and need for system performance risk assessment, inspire interest in the development and validation of mechanism-based models of SCC.  The objective is to characterize and quantify input data that, upon completion, will enhance accuracy of life predictions for structural integrity studies.  An analytical and experimental investigation is ongoing to develop this capability, with a focus on SCC behavior associated with low temperature sensitizations.  High resolution stress corrosion crack growth measurements were optimized for specimens with differing degree of sensitization (DoS) under full immersion in aqueous chloride solution. Results establish a correlation between crack growth rate and applied stress intensity, as well as with DoS.  The maximum crack growth rate in stage II SCC of sensitized 5083 is two orders of magnitude greater than crack growth rates due to hydrogen diffusivity in aluminum. Substantial crack growth is observed prior to the stress intensity threshold, and has been attributed to IGC, perhaps stress assisted. The challenge is to characterize crack growth rates varying with microstructural features at the micrometer length scale, while quantifying the effect of long time-low temperature (< 100 ^oC) sensitization. These results, in combination with an initial damage size distribution based on IGC penetration and spreading, provide the foundation for a probabilistic fracture mechanics model of stress corrosion crack evolution emanating from IGC sites for a given stress and DoS.  The fundamental understanding of the interacting damage modes from this study will support prognosis of fitness-for-mission.